How long alt text should be

The number everybody quotes appears in no standard. It is still a good default, and knowing why is the difference between following it and being ruled by it.

Where 125 came from

Search for a length and you will be told 125 characters, usually without a source. The figure is real but it is not a specification. It comes from the behaviour of older screen-reader and browser combinations, where the alt value was read in one pass and longer strings could be cut, and from a period when a good deal of assistive technology had a practical buffer somewhere near that mark.

Almost none of that is still true. Current versions of NVDA, JAWS and VoiceOver read the whole attribute, however long it is. What survives is not a technical limit but an attention limit, and that turns out to be the more useful constraint: a listener is getting this as speech, in one uninterrupted run, with no way to skim it and no way to go back a few words without going back to the start.

So 125 is a good default for the reason most good defaults are good. It is roughly one spoken sentence. It is right more often than it is wrong. And when it is wrong, it is wrong in a direction you can see coming.

What actually truncates, and where

Screen readers

In current versions, nothing. The historical cut-off figures that circulate — 125 here, 255 there — describe combinations that were already being retired years ago. Writing to the old limit costs a listener nothing; believing you must is what costs you a chart description.

Search and social previews

This is where length genuinely bites, and it is not accessibility. Systems that lift image text into a snippet or a card cut at their own limits, and those limits are neither published nor stable. If the alt text is doing double duty as marketing copy, that is a reason to keep the first clause self-contained — not a reason to truncate the whole thing.

Content management systems

The real truncation risk in practice. Plenty of platforms cap the alt field at some arbitrary length and silently drop the rest on save. It is worth testing once, on your own stack, with a deliberately long string.

Four cases that should run long

Every one of these is an image where the page’s meaning is inside the picture rather than beside it, which is the only argument for length that holds up.

Short, then long enough

As writtenBar chart of sales.
InsteadBar chart of quarterly sales, rising steadily through the year apart from a sharp fall in the third quarter. Full figures follow in the text below.
A chart whose point is the shape of the data needs the shape stated. Naming it and stopping leaves the listener knowing a chart exists and nothing about what it argues.
As writtenDr Ambedkar.
InsteadB. R. Ambedkar in a dark three-piece suit, holding a bound volume, photographed seated against a plain backdrop.
A portrait used as evidence of how someone was presented needs the presentation described. A portrait used to say who wrote a book does not, and the same photograph gets different alt text on different pages.
As writtenInstructions diagram.
InsteadThree-step diagram: remove the cover plate, disconnect the two ribbon cables, then lift the drive out of its cage.
If the steps are only in the picture, a short alt has removed them from the page. The alternative to a long alt here is not a short alt — it is a long description in the body text, which is usually better still.
As writtenScreenshot of the settings page.
InsteadThe settings page with the privacy tab open and the location toggle switched off, which is the state described in the paragraph above.
A screenshot in a walkthrough is there to show one state. Naming the screen is not describing the state, and the state is the entire reason the image is on the page.

A working rule that is not a number

Write what a reader would miss without the picture, then stop. If that comes to forty characters, forty is correct and padding it to reach a guideline makes the page worse. If it comes to two hundred, ask one more question before shipping: is this really one image doing a lot, or is it an image that has outgrown an attribute and wants a long description in the body instead?

That second question is the one the character count is a poor proxy for. Length is a symptom. What matters is whether the words belong in an attribute that gets read once, in order, with no way back.

Where to take this next

The other written pages